As it is known, a television signal comprises both audio and video that shall be synchronized when played.
DVB (Digital Video Broadcasting) standards provides for broadcasting transport streams each comprising a plurality of channels (services). Each transport stream comprises a plurality of elementary packets transporting audio, video and data.
Each frame of the television signal can require a different bandwidth for its transmission, e.g. I-frames are less compressed then P and B-frames. Each frame can therefore be divided in a different number of elementary packets and transmitted in one or more transport streams.
As a result, audio and video are not always transmitted in the same transport stream; actually audio can arrive up to 3 s later then the respective video.
When a user changes TV channel, audio and video are displayed only when the television appliance has been able to receive both of them. The result is that when a user changes channel, a black screen is displayed for few seconds or milliseconds depending on the transmission delay between audio and video.
If time delay is long, often user can change channel thinking that no signal is available; this often happens during zapping, wherein the user changes channel very rapidly to have an overview of what's on TV.
WO2009/047273 faces the problem of channel zapping time and discloses a method for an early start of audio-video rendering.
WO2009/047273 discloses several methods wherein after video stream reception, video rendering is started with a speed which is slower than the normal rendering speed. Rendering speed is then accelerated until a video buffer is filled and video is rendered at normal speed.
According to the different embodiments disclosed by WO2009/047273, audio can be rendered synchronized or desynchronized to the video during video speed acceleration. In case audio is rendered desynchronized to video, this can be annoying for the user. In case audio is rendered synchronized to video, an audio pitch algorithm is necessary in order to compensate for audio distortion introduced by the reduced video speed; such an algorithm can be expensive and not that efficient in case of audio rendering at very slow speed. If audio pitch algorithm is not available, according to WO2009/047273 audio is not rendered until video can be rendered at normal speed; this solution has the drawback that audio that could be rendered, is not rendered and the user therefore loses information carried by the audio.